The present invention relates to a catalyst for propylene ammoxidation to acrylonitrile. More particularly, the present invention relates to a fluidized-bed catalyst for propylene ammoxidation to acrylonitrile.
The acrylonitrile is a basic organic chemical material. It has been widely used in chemical industry. At present, producing acrylonitrile mainly use the process of propylene ammoxidation. In the process, catalyst makes important effect on propylene conversion, acrylonitrile selectivity, and so on. Nowadays, the most popular catalyst used for the propylene ammoxidation is a fluidized-bed catalyst. In order to get high activity and high selectivity. The catalyst have already been made a series of improvements through long-lasting research. Most of these improvements mainly focused on the active component of the catalyst and the mixture ratio of the active component.
It has been well know that the balance between acrylonitrile output and market demand is gradually being formed. Nowadays, acrylonitrile manufacturers are paying more attention to increasing acrylonitrile output and reducing starting material consumption of existing plants rather than setting up new plants. Utilizing a new catalyst with excellent performance replace old one to innovate existing plant can eliminate the so called xe2x80x9cbottle neckxe2x80x9d in the production process, increase 50-80% production capacity and save lots of cost in comparison with setting up new plant. The economic efficiency is considerable high.
However, two problems will occur in the process of making technical innovation. One is reaction pressure go up, another is catalyst amount is beyond a certain limitation. Therefore, it is required that the new catalyst should have higher capacity for loading propylene and higher capacity for bearing reaction pressure.
The reaction pressure of fluidized-bed reactor comes from a series of equipment resistance from the outlet of reactor to the top of absorbing tower, such as heat exchanger, tower and pipeline. Because of increasing production capacity, it consequently makes the amount of flowing materials apparently increase at reactor outlet and results in the increase of resistance. In addition, it makes further increase of resistance force that more heat exchanging equipment need to be used due to the deficient heat transferring area. In order to meet the requirement of the environmental protection, the waste gas of reaction from the absorbing tower top is not allowed to discharge directly into atmosphere and it will be transferred to waste-gas burning unit to burn. Thus, the pressure of the top of absorbing tower is to be raised if a blowing machine has not been used. Due to the above-mentioned reasons, at present, the running pressure of reactor is higher than the design pressure by 0.5-1.0 times. It is to reach up to above 0.08 Mpa.
Actually, the second problem is a catalyst load, namely WWH. It means the weight of proprlene feed per unit weight of catalyst per hour. With increase of the feedstock entered into the reactor, the fluidizing height of catalyst may exceed the height of cooling water pipe if the catalyst load is unchanged. At the same time, the reaction linear velocity of the reactor also should be outstandingly raised. These two changes may lead the increase of reactor dilute phase temperature, resulting in the increase of the amount of carbon dioxide formed and the decrease of the acrylonitrile selectivity. Accordingly, the catalyst with higher WWH can settle the above-mentioned problem.
Theoretically, the capacity of absorbing propylene should be improved as the increase of the catalyst WWH. However, there is not any theory indicate a certain relationship between the metal elements used in the catalyst and capacity of absorbing propylene.
Various methods were proposed in the prior art for solving the aforementioned problems. For example, Chinese patent CN1021638C disclosed the catalyst, which had a composition represented by following general formula:
AaBbCcNidCoeNafFegBihMiMojOx
Wherein,
A represents K, Rb, Cs, Sm and Tl;
B represents Mn, Mg, Sr, Ca, Ba, La and rare earth element;
C represents P, As, B, Sb and Cr
M represents W, V
The catalyst had a higher yield of acrylonitrile, but lower propylene load. The yield of acrylonitrile of the said catalyst would be considerably decreased under higher reaction pressure. Based upon further research done by the present inventor, it is found that the component B and M have effect on the running load of the catalyst and on its properties under high pressure. Some metal elements of component B will adversely affects on the increase of running load of catalyst as well as the catalyst properties under high pressure, so these metals are not suitable to be used in the catalyst running under higher pressure and higher load. Besides, in the above-mentioned Chinese patent CN1021638C, it was provided that the sum of i and j in the catalyst composition should be 12, namely it was a constant. Under the rule, the component Mo was decreased as the component M was increased. It would result in the adverse effect on the acrylonitrile yield. The catalyst composition of the present invention is not limited by the rule.
The object of the present invention is to provide a new fluidized-bed catalyst for propylene ammoxidation to acrylonitrile, which can overcome the problem that the fluidized-bed catalyst for propylene ammoxidation to acrylonitrile is not suitable for higher reaction pressure and higher load. The catalyst of the present invention can achieve higher acrylonitrile even under higher reaction pressure and higher load.
The object of the present invention can be realized by a new fluidized-bed catalyst is used in the process of propylene ammoxidation to crylonitrile. The catalyst comprises silica as carrier and a catalytic composition represented by following general formula:
AaBbCcGedNaeFefBigMohOx
Wherein
component A represents at least one element selected from the group consisting of Li, K, Rb, Cs, Sm, In and Tl;
component B represents a mixture of Pr plus W or a mixture of Pr plus W and at least one element selected from the group consisting of P, Sb, Cr, Ce, As, B, Te, Ga, Al, Nb, Tb, La and V;
component C represents at least one element selected from the group consisting of Ni, Co, Sr, Mn, Mg, Ca, Zn, Cd, Cu and mixtures thereof;
a is a number of from 0.01 to 1.5;
b is a number of from 0.01 to 3.0;
c is a number of from 0.1 to 12.0; preferably from 2 to 10;
d is a number of from 0.01 to 2.0; preferably from 0.01 to 1.0;
e is a number of from 0.01 to 0.7; preferably from 0.05 to 0.5;
f is a number of from 0.1 to 8; preferably from 1.0 to 3.0;
g is a number of from 0.01 to 6; preferably from 0.1 to 2.0;
h is a number of from 12 to 14.5;
the atom value of Pr is a number of from 0.01 to 0.75;
the atom value of W is a number of from 0.01 to 1.0; and
x is a number of oxygen atoms required to satisfy the valence requirement of the other element in the catalyst;
The content of silica as carrier is 30-70 wt %; preferable 40-60 wt %.
In a preferred embodiment of the invention, the component C is selected from the group consisting of Mg and a mixture of Mg with other metals. In another preferred embodiment of the invention, the component B is selected from the group consisting of a mixture of Pr plus W plus Sb and a mixture of Pr plus W plus Sb with other metals, and component C is selected from the group consisting of Sr and a mixture of Sr with other metals. In another preferred embodiment of the invention, component B is selected from the group consisting of a mixture of Pr plus W plus Ce and a mixture of Pr plus W plus Ce with other metals, and component C is selected from the group consisting of Mg and a mixture of Mg with other metals. In another preferred embodiment of the invention, component B is selected from the group consisting of Pr plus W plus Cr plus La and a mixture of Pr plus W plus Cr plus La with other metals, and component C is selected from the group consisting of Mg and a mixture of Mg with other metals.
The fluidized-bed catalyst of the present invention is named as Moxe2x80x94Bi type catalyst because Mo and Bi constitute the main active components in this type of catalyst. Fe can form various new compounds by mixing it with Moxe2x80x94Bi, so as to make the catalyst have higher activity and selectivity. Besides, it can not only speed up to reach the balance of oxidation-reduction reaction of the catalyst and maintain the stability of the catalyst structure, but also prevent Moxe2x80x94Bi system from being permanently deactivated under the condition of low oxygen.
In the catalyst of the invention, A type of metals are mainly used for lessening the surface acidity of the catalyst and enhancing the selectivity of propylene ammoxidation to acrylonitrile. The B type of metals are added as an assistant component of active phase in the catalyst and they usually form a structure of heteropolyacid salt with active component. The C type of metals is mainly used as promoter. They can make the catalyst have a proper oxidation capability so as to prevent the reaction from excessive oxidation, therefore the catalyst performances of both activity and selectivity can be enhanced.
In the present invention, Na added into the catalyst can not only strengthen resistance attrition, but also enhance both the activity and selectivity when Na is used together with Ge.
There is no special requirement on the process of preparing the catalyst of the invention. The catalyst of the invention may be prepared by various conventional methods. The preparing process may comprises: the selected components are mixed to form a solution, then mixed with catalyst carrier to obtain a slurry, the resultant slurry is spray dried to solid particles and calcined to obtain the catalyst. The aforementioned slurry is preferably prepared with the process described in Chinese patent CN1005248C. The said process is as follows: the various components of the catalyst in the form of aqueous solution and the carrier are added into the equipment for manufacturing catalyst in the order as indicated in the below mentioned examples of the present invention. Alternatively, some of the components are premixed with Mo compound to form molybdate, then mixed with other components. Due to the excellent resistance of the catalysts to attrition, it is not necessary for the carrier to be added into the catalyst of the invention by multiple times. So preparation of the catalyst is greatly simplified.
The starting materials used for preparing the catalyst of the invention may be selected by a manner as follows:
Mo component as the starting material used for preparing the catalyst can be selected from a group consisting of molybdnum oxide, ammonium molybdate or molybdnum salts.
Na component as the starting material used for preparing the catalyst can be selected from a group consisting of sodium nitrate, sodium hydroxide, sodium silicate or any decomposable Na compound. There is no special requirement on the order of adding Na component into the slurry. Na component can be added into the solution of ammonium molybdate, the solution of nitrate or into the ammonia-stabilized silica sol used as carrier. Alternatively, Na component also can be added into ammonia-stabilized silica sol in the plant for manufacturing silica sol in advance by requiring the manufacturer of silica sol to do so. If such is the case, the weight ratio of SiO2/Na2O should be within the range from 150 to 550, preferably from 200 to 400.
To introduce into the catalyst, one preferably employs respectively the acids of P, As and B or the ammonium salts thereof. To introduce W into the catalyst, one can employ ammonium tungstate or tungsten oxide. To introduce V into the catalyst, one can employ ammonium metavanadate. To introduce Ge into the catalyst, one can employ the oxides of Ge. To introduce Cr into the catalyst, one preferably employs chromium trioxide, chromium nitrate or the mixture thereof. To introduce Sb into the catalyst, one can employ antimony trioxide, antimony pentoxide and antimony halide or antimony sol which can be hydrolyzed to form antimony oxide. To introduce Ce, Pr, Rb, La and Tb into the catalyst, one can employ nitrate or salts, which can be decomposed to form oxides thereof.
To introduce into the catalyst, one can respectively employ the oxides of Bi, Fe and Sr or the salts thereof which can be decomposed to oxides, most preferably employs water soluble nitrates. To introduce rest of metal components other than aforementioned elements into the catalyst, one can employ nitrates of these metals, oxides thereof or salts thereof which can be decomposed to oxides, most preferably employs water soluble nitrates thereof.
In the method for preparing the catalyst of the invention, the compounded slurry is heated and concentrated till the solid content reaching to 47-55 wt %, then spray dried. The spray drier used in the method can be one selected from a group consisting of pressure-type, parallel flow type or centrifugal rotating-disc drier, preferably is the centrifugal rotating-disc drier which can ensure the catalyst prepared have a good distribution of size.
Calcination of the catalyst can be divided into two steps i.e. the decomposition of the salts of various elements in the catalyst and the calcination at high temperature. Temperature for decomposition is preferably 200-300xc2x0 C., the time for calcination is from 20 minutes to two hours. The decomposition and calcination can be carried out respectively in two calcinators. It is also possible that a calcinator is divided to two sections respectively for decomposition and calcination, or the decomposition and calcination are carried out in a continuous rotating calcinator. A certain amount of air should be introduced into the process of decomposition and calcination of the catalyst to prevent the catalyst from being reduced excessively.
Propylene, ammonia and molecular oxygen employed in the process of producing acrylonitrile by using the catalyst of the invention is same as that employed in the ammoxidation process by using other catalyst. Although, the content of low molecular saturated hydrocarbon in propylene as raw material will have no effect on the reaction, the concentration of propylene is preferably more than 85% (mole) from the point of view of economics and safety. The ammonia used can be liquid ammonia with fertilizer grade. The molecular oxygen required for reaction can be pure oxygen, oxygen rich air or normal air, while preferably is normal air from the point of view of economics and safety.
The molar ratio of ammonia to propylene as feedstock added into the fluidized-bed reactor is within the range from 0.8 to 1.5, preferably from 1.0 to 1.3. The molar ratio of air to propylene is within the range from 8 to 10.5, preferably from 9.0 to 9.8. The molar ratio of air to propylene also can be up to 11 if it is required due to some reasons in the running and there is still no any adverse effect on the reaction. However, the amount of excessive oxygen in the reaction gas had better be no more than 7% (volume) by taking the consideration of safety, preferably no more than 4% (volume).
When the catalysts of the present invention are used in a fluidized-bed reactor, the reaction temperature is 420-470xc2x0 C., preferably 425-450xc2x0 C. The catalyst of the present invention is suitable for running under the condition of high-pressure, high-load. Hence, the reaction pressure in the production unit can be up to over 0.08 Mpa, for example, 0.08-0.15 Mpa. If the reaction pressure is below 0.08 Mpa, there is no any disadvantageous effect on the reaction while the yield of acrylonitrile can be increased further.
The propylene load of the catalyst of the invention (WWH) is 0.06-0.15 hrxe2x88x921, preferably 0.07-0.1 hrxe2x88x921. If the load is too low, it will not only cause the waste of catalyst, but also increases the amount of carbon dioxide formed and decreases the selectivity. If the load is too high, it has not practical value. It is the amount of catalyst is so little that make the transferring heat area of cooling water tube in the catalyst layer is less than the area required to remove the reaction heat. As a result, the reaction temperature will be out of control.
The evaluation of the activity of the catalyst of the present invention is determined using a fluidized-bed reactor having an inner diameter of 38 mm. The amount of the catalyst provided to the reactor is 440-550 g, the reaction temperature is 430-440xc2x0 C., the reaction pressure is 0.14 Mpa, the molar ratio of propylene to ammonia to air in the feedstock is 1:1.2:9.5-9.8, the propylene load of the catalyst (WWH) is 0.085-0.090 hrxe2x88x921.
The propylene conversion and the selectivity of acrylonitrile and the yield of acylonitrile are respectively calculated in accordance with the following equations.             propylene      ⁢              xe2x80x83            ⁢      conversion      ⁢              xe2x80x83            ⁢              (        %        )              =                            moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propylene          ⁢                      xe2x80x83                    ⁢          converted                          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propylene          ⁢                      xe2x80x83                    ⁢          fed                    xc3x97      100      ⁢      %                  acrylonitrile      ⁢              xe2x80x83            ⁢      selectivity      ⁢              xe2x80x83            ⁢              (        %        )              =                            moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          acrylonitrile          ⁢                      xe2x80x83                    ⁢          produced                          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propylene          ⁢                      xe2x80x83                    ⁢          converted                    xc3x97      100      ⁢      %                  once      ⁢              -            ⁢      through      ⁢              xe2x80x83            ⁢      yield      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      acrylonitrile      ⁢              xe2x80x83            ⁢              (        %        )              =                            moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          acrylonitrile          ⁢                      xe2x80x83                    ⁢          produced                          moles          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          propylene          ⁢                      xe2x80x83                    ⁢          fed                    xc3x97      100      ⁢      %      
The content of silica in the catalyst is 40-60 wt %.
The following non-limiting examples will serve to illustrate the present invention in more details. However, it should be understood that these examples are only illustrative and should in no way limit the scope of the present invention. It will be apparent that numerous variations, modifications, and alternative embodiments are to be regarded as being within the spirit and scope of the present invention as claimed.